(1) Field of the Invention
The present invention relates to a filler for a heat-sensitive recording paper. More particularly, the present invention relates to a filler for a heat-sensitive recording paper which comprises a finely divided amorphous silicate having novel characteristics. Furthermore, the present invention relates to a heat-sensitive recording paper comprising this filler.
(2) Description of the Prior Art
A heat-sensitive recording paper comprising a support such as paper and a recording layer formed thereon, which comprises a dispersion of a coloring agent such as a leuco dye and a color developer capable of forming a color on contact with the coloring agent in the hot state, such as a phenol, in a binder has been widely used for facsimile, printers, data communication, computer terminals, measuring devices, passometers, copying machines and the like while using a thermal head, a hot pen, an infrared ray lamp, a laser or the like as a heat source.
A heat-sensitive recording paper of this type is defective in that when recording is carried out by bringing a recording layer into contact with a recording head or the like, the components contained in the recording layer are fused and adhere to the recording head or the like to cause such troubles as scum adhesion and sticking.
Various fillers have been incorporated into recording layers so as to eliminate this disadvantage. Namely, it has been known from old that calcium carbonate, kaolin, talc, alumina and titanium dioxide are incorporated. Recently, incorporation of a hydrous aluminum silicate mineral (Japanese Patent Application Laid-Open Specification No. 72992/81), amorphous synthetic aluminum silicate (Japanese Patent Publication No. 19035/82), wollastonite or calcium silicate (Japanese Patent Application Laid-Open Specification No. 41995/82), an alkaline earth metal salt (Japanese Patent Application Laid-Open Specification No. 80095/82) and aluminum hydroxide (Japanese Patent Application Laid-Open Specification No. 14093/82) has been proposed.
When these inorganic fillers are used for heat-sensitive recording papers, various limitations are imposed on the properties thereof. In the first place, in order to prevent the adhesion of scum, the filler used should have a certain oil absorption, that is, a large bulk. The second problem is how to prevent the background coloration (background contamination or back ground fogging) of the recording layer. In the case of a filler having a relatively large surface activity, the recording layer is colored in an inherent hue before the recording and a clear image cannot be obtained. Furthermore, the background is colored during the storage after the recording, and the storability or life of a print is degraded. In the third place, when a filler is incorporated into the recording layer, it should show an excellent abrasion resistance. For example, the filler should not inhibit a smooth relative movement between a recording head and a recording paper or should not abrade the recording head or recording layer.
Conventional fillers for heat-sensitive recording layers fail to simultaneously satisfy all of these requirements. For example, a filler having a large oil absorption generally has a large surface activity and the background coloration is readily caused.